Induction machines, both motors and generators, are well known and widely used in electrical power systems for many applications. As is well known, a standard induction machine utilizes the commonly referred to transformer-like action to produce electricity or mechanical power from currents induced in the rotor windings from rotating fields produced by the stator windings. To obtain any useful work, the rotor must turn slightly slower or slightly faster than the stator's rotating magnetic field. If the rotor increases or decreases in speed, the rotor current will increase. If the rotor should stop entirely, large short circuit currents will be produced unless external resistance or reactance is incorporated into the rotor winding circuits. Thus, induction machines have been categorized generally as having limited speed ranges for practical application and as having very large rotor currents outside of the limited speed range. Rather elaborate speed control schemes using wave shaping or chopping have been utilized to overcome these limitations in operation but generally result in overheating of motor windings or creation of radio frequency noise and poor electrical load characteristics. Oftentimes these starting systems tended to reduce the efficiency of the motor design and construction.
It would be desirable to have an induction machine which retains a sine wave operation throughout a full range of speed control without creating overheating in the rotor windings or other deleterious effects thereto. Preferably such a machine would operate with practical rotor currents at low speeds, and, in motor operations, still produce sufficient torque to drive the machine.
One application receiving widespread attention at this time which would find these characteristics particularly attractive is in the operation of wind turbines to produce or generate electricity. Since the wind turbine speed is normally dependent upon wind velocity, fairly elaborate mechanical schemes have been proposed or developed to adjust the wind turbine chracteristics to reduce the turbine speed fluctuations with wind velocity changes or to modify the drive systems rotational speed to a relatively constant level regardless of turbine velocity. These arrangements are both expensive and often mechanically inefficient. In other wind turbine arrangements, the turbine is used to drive a direct current generator and then electrical means are utilized to convert the direct current energy to an alternating current. These latter schemes require rather costly and oftentimes inefficient electrical systems or produce deleterious harmonics in the power output which is either unusable by conventional alternating current systems or must be filtered from the power lines. It would be desirable to provide an alternating current machine which could generate 60 cycle sinusoidal electrical power directly onto the line power system without further conversion and regardless of turbine speed, above some minimum turbine velocity.
In addition, there are some wind rotor or turbine systems in which it would be desirable that the wind turbine be broght up to an operating speed by use of the same machine which is used to generate the electrical power, such as in those wind turbines which are not self-starting. Such an induction machine could then drive the wind turbine to an operating speed when the wind energy reached a level which sould support turbine action and then automatically be converted to a generator so long as the wind energy remained above the same desired threshold determined by the wind turbine characteristics and the generating system. As the wind velocity varied, and consequently the rotational speed of the wind turbine, such an induction machine could then continue to provide 60 cycle sine wave electrical energy to a power grid.